The Shield has done remarkably well in a market where there were already so many choices: LC9, Nano, PPS, Solo, Kel-Tecs, Kahrs, Diamondback, etc, etc.
The Shield so far seems to be, at least on the average, much more reliable than other subcompacts. Also, I felt that S&W paid very close attention to QC on the first run of Shields that went on sale after the announcement. That really helped them get the gun off to a good start.
There have been a few issues such as the 40 dropping the magazines, but overall it has done well. I do have to say S&W could have been a bit more progressive at resolving the mag drop issue though.